Existing methods formulate the problem as sequence labeling [1]} or field extraction [2]}, where they define a fixed set of items of interest (referred as fields) and train models that only extract values of the pre-defined fields. There are at least two limitations of this formulation. First, forms are very diverse and it is impossible to cover all the items of interest using a fixed set of fields. Second, their models are very domain-specific and hard to be utilized for different form types. For example, an invoice field extractor may not be able to process resumes, since different fields are expected for these two form types.
